The present invention relates to an apparatus for measuring air flow rates and, more particularly, to an air flow measuring apparatus suitable for metering intake air flow rates of an internal combustion engine for a vehicle.
There is a heat generating resistance type air flow measuring apparatus which is a kind of air flow measuring apparatus. The air flow measuring apparatus has a specific property by which it is possible to directly meter mass flow rates of a fluid to be metered and has been widely used as an intake air flow measuring apparatus for an internal combustion engine.
In general internal combustion engines, for example in an engine in which the number of cylinders is 4 or less, when the engine runs into an operational condition of a low engine speed and a high load, a pulsation amplitude of an intake air flow becomes large, and in some cases the intake air flow becomes pulsation flow accompanied by a partial backward flow. As a result, a conventional flow meter has a lowered measurement precision. Therefore, a correction method therefor is proposed in, for example.
Further, as a method of correcting errors caused by such an intake backward flow, for example, JP A 59-17371 proposes a correction method without detecting a direction of an air flow. According to this method, an alternating flow component of the air flow is detected, and correction is applied to the whole of the wave, whereby the error is nullified.
Further, for example, JP A 9-88711 proposes a correction means by software in which an air flow rate is detected as voltage and linearized (unit conversion). Then, advance-processing is effected to thereby detect a backward flow condition and correct an error.
On the other hand, JP A 6-265565, for example, proposes a construction in which a flow sensor is used as a flow speed meter. The output from the sensor is converted into digital values by an A/D convertor, and the characteristics are adjusted by a functional equation of a memory having prescribed factors recorded and displayed on a display. Alternatively, a construction is proposed in which the output is converted into analogue values by a D/A convertor (digital analogue convertor) and outputted as voltage.
Further, for example, JP A 8-94406 proposes a method of reducing an influence of flow rate fluctuation due to pressure fluctuation, using a construction similiar to the above. According to the method, after the output from the flow sensor is converted into digital values by an A/D convertor, they are linearized, and then an average value thereof is calculated, whereby a flow rate without errors can be displayed.
Further, each prior art system has a system construction which is closed as a flow meter.
The above-mentioned prior art systems do not take into sufficient consideration the case where a backward flow occurs in the air flow to be metered, so that a problem is left in measurement precision.
Here, backward flow means an air flow in the opposite direction to an intake air flow in the intake air duct of an internal combustion engine, and the backward flow is caused for the following reasons:
In general internal combustion engines, valve opening time overlaps between an intake valve and an exhaust valve and, as a result, an air flow flowing backward from the exhaust valve side to the intake valve side takes place in the overlapping time period, whereby the air flow in the opposite direction occurs in the intake duct.
A kind of flow meter, for example, a general heat generating resistance type air flow measuring apparatus, has the property of outputting a positive signal corresponding to an absolute value of a flow speed, irrespective of the direction of flow of a fluid to be metered.
Therefore, when such a flow meter meters an air flow rate including a backward flow, as a result, a flow rate which has a flow rate of the backward flow added to a flow rate of an air flow in the direction to be metered, that is, a normal flow rate, is output as a metered value.
In this case, since the backward flow also is detected and outputted as the normal flow rate, a signal of larger flow rate than a true average air flow rate is outputted. A measuring error thereby reaches 30-100% and measurement precision can not be maintained.
Next, in the heat generating resistance type air flow measuring apparatus, an air flow rate Q is expressed by the following equation (1) known as King""s equation:
Ih2xc2x7Rh=(C1+C2xc2x7Q)xc2x7(Thxe2x88x92Ta)xe2x80x83xe2x80x83(1)
where Ih is a current value of a heat generating resistor, Rh is a resistance value of the heat generating resistor, Th is a surface temperature of the heat generating resistor, Ta is a temperature of air, Q is an air flow rate and C1 and C2 are constants determined by the specification of the heat generating resistor.
As for output of measurement of a flow rate, it is general to detect current flow Ih of the heat generating resistor by converting the current into voltage value V by voltage drop of the resistor. In an engine control unit, the voltage value V is converted into a flow rate Q from the equation (1) for engine control.
Here, for a probe (a detecting portion) of a heat generating resistance used for measuring an intake air flow rate of a vehicle, it is necessary to use a thick wire to some extent in order to secure reliability when a resistance wire is used for the probe, for example. As a result it can not be avoided for the probe to have heat capacity to some degree.
In this case, delay in response to dynamic flow variations such as pulsations takes place, and when pulsations appear in an engine intake air flow, precise pulsation waves can not be measured, so that there is a problem that errors are apt to be contained in the measurement.
Further, in vehicles, when sensor signals are sent to a control unit for controlling an internal combustion engine, in view of noises from other devices and apparatus, it is considered preferable from a viewpoint of S/N (a ratio of signal to noise) to take interface with signals which have a non-linear characteristic to flow rates without converting them so as to be linear.
Therefore, it is necessary to enable an air flow measuring section to detect an air flow rate with a high response using a heat generating resistance probe of a small heat capacity such as a semi-conductor probe and, in the engine control unit side, to effect high speed sampling of signals inputted from the air flow measuring section.
However, it is general for usual engine control units not to be provided with such a high processing ability and, therefore, it is difficult to cope with such a high response and there is a problem that measurement precision is lowered by delay in sampling.
An object of the present invention is to provide an air flow measuring apparatus (or air flow measuring device) which has a little limit to sampling cycles and in which the measurement precision lowering due to air flow pulsations and noises is sufficiently suppressed.
The above-mentioned object is achieved by a heat generating resistance type air flow measuring apparatus for measuring air flow rates using a heat generating resistor arranged in an air flow passage, which air flow measuring apparatus comprises a first converting means for converting voltage values detected by the heat generating resistor into flow rate values by a first conversion equation (first calculation equation) defined in advance, a filtering means for smoothing output from the first converting means, and a second converting means for converting output from the filter means into voltage values corresponding to the flow rate values by a second conversion equation (second calculation equation) defined in advance, wherein output of the second converting means is taken out as flow rate detection signals.
A means can be provided for recovering the response of a sensor by voltage value before converting into flow rates the voltage signals of the sensor which detected air flow rates.
Further, a means can be provided for switching a plurality of processing means for smoothing, according to pulsation characteristics of air flow rates.
Even if air flow pulsates, measurement precision of an average air flow rate can be raised without losing the signal detection characteristics inherently provided in the sensor.